AF-ON problems

I just recently discovered the AF-ON setting on my D40 and D50 cameras after reading a review of the D3 of all things (it's a dedicated button on the D3). I found the AF-ON to be interesting and it prompted me to go exploring in my camera's menus to find similar functionality.

What I would like to do is to be able to use the AF-L/AF-L button to drive the focus and separate it from the shutter operation. I'd like to be able to focus on an object, recompose and then fire off several captures without having the camera re-focus between shots.

The problem that I am having is that when I us the AE-L/AF-L button to activate the focus on an object (press and release AE-L/AF/L) it will work just fine until I recompose the image. As soon as I recompose and the focus point moves to the background (which is much further away) the shutter refuses to fire. I've played with combinations of the above settings but have had no luck. If I move the center focus point back to the object that I focused on earlier it will sometimes fire just fine.

Should I refocus on the new background object by pressing AE-L/AF-L the shutter will fire.

If I hold the AE-L/AF-L button during recomposition then the first image is correctly focused but the next is focused on the background. This problem renders this functionality almost useless for what I'm after. I've missed shots while experimenting with this function.

I don't imagine the engineers at Nikon are this stupid, I'm more inclined to think it's me. lol. Assuming I'm doing something wrong here I'd love some direction from the forum.

I find it hard to believe that it's ever a good idea for a camera to refuse to fire, especially in a manual mode. If the photographer has set things up the shutter should fire unless the camera fears that it's going to break. That's what I've done here; I determined the shutter speed, aperture and ISO and by pressing the AE-L/AF-L button I've set the focal distance that I want. Dammit camera, just fire!

I know I could use manual focus but that's not what I'm after. Actually using the AE-L/AF-L with my right thumb and then immediately using the AF/M dial with my left middle finger isn't too bad on the D50 but it's not an option on the D40 at all.

I know the post is a little longish, I have tried to be somewhat detailed. I haven't been able to find anything pertinent by searching the archives. So many thanks in advance for your help in this matter.